Molding is a process by virtue of which a molded article can be formed from molding material (such as Polyethylene Terephthalate (PET), Polypropylene (PP) and the like) by using a molding system. Molding process (such as injection molding process) is used to produce various molded articles. One example of a molded article that can be formed, for example, from PET material is a preform that is capable of being subsequently blown into a beverage container, such as, a bottle and the like.
A typical injection molding system includes inter alia an injection unit, a clamp assembly and a mold assembly. The injection unit can be of a reciprocating screw type or of a two-stage type. Within the reciprocating screw type injection unit, raw material (such as PET pellets and the like) is fed through a hopper, which in turn feeds an inlet end of a plasticizing screw. The plasticizing screw is encapsulated in a barrel, which is heated by barrel heaters. Helical (or other) flights of the screw convey the raw material along an operational axis of the screw. Typically, a root diameter of the screw is progressively increased along the operational axis of the screw in a direction away from the inlet end.
As the raw material is being conveyed along the screw, it is sheared between the flights of the screw, the screw root and the inner surface of the barrel. The raw material is also subjected to some heat emitted by the barrel heaters and conducted through the barrel. As the shear level increases in line with the increasing root diameter, the raw material, gradually, turns into substantially homogenous melt. When a desired amount of the melt is accumulated in a space at discharge end of the screw (which is an opposite extreme of the screw vis-à-vis the inlet end), the screw is then forced forward (in a direction away from the inlet end thereof), forcing the desired amount of the melt into one or more molding cavities. Accordingly, it can be said that the screw performs two functions in the reciprocating type injection unit, namely (i) plasticizing of the raw material into a substantially homogeneous melt and (ii) injecting the substantially homogeneous melt into one or more molding cavities.
The two stage injection unit can be said to be substantially similar to the reciprocating type injection unit, other than the plasticizing and injection functions are separated. More specifically, an extruder screw, located in an extruder barrel, performs the plasticizing functions. Once a desired amount of the melt is accumulated, it is transferred into a melt accumulator, which is also sometimes referred in the industry as a “shooting pot”, the melt accumulator being equipped with an injection plunger, which performs the injection function.
U.S. Pat. No. 6,241,932 issued to Choi et al. on Jun. 5, 2001 discloses a method and system of operating a two stage injection molding machine wherein movement of the injection plunger in the shooting pot is coordinated with movement of the plasticizing screw and melt flow into the shooting pot such that the plunger provides minimal resistance to the melt flow into the shooting pot while avoiding the production of voids or air inside the melt. The undesired shear forces to which the melt is exposed are thus reduced, correspondingly reducing the melt degradation products which would otherwise result.
U.S. Pat. No. 6,514,440 to Kazmer, et al. issued on Feb. 4, 2003 discloses an injection molding apparatus, system and method in which the rate of material flow during the injection cycle is controlled. According to one preferred embodiment, a method of open-mold purging is provided in an injection molding system including a manifold to receive material injected from an injection molding machine. The method includes the steps of selecting a target purge pressure; injecting material from the injection molding machine into the manifold; and controlling the purge pressure to substantially track the target purge pressure, wherein the purge pressure is controllable independently from the injection molding machine pressure.
U.S. Pat. No. 4,311,446 to Hold et al. issued on Jan. 19, 1982; U.S. Pat. No. 4,094,940 to Hold on Jun. 13, 1978; U.S. Pat. No. 3,937,776 to Hold et al. on Feb. 10, 1976; and U.S. Pat. No. 3,870,445 to Hold et al. on Mar. 11, 1975 each teaches a method and apparatus for controlling the parameters of injection molding processes in a machine having a barrel with a plasticating chamber and a screw, rotatably and slidably disposed in said chamber, hopper means adjacent one end of said chamber communicating therewith and nozzle means disposed in the other end of said chamber communicating with a mold. Control of the injection molding process is achieved through an event recognition philosophy by sensing screw position, screw injection velocity, melt temperature, comparing the values at certain instances during the work cycle with known or desired values and using these values, changes of values and differences of values to monitor and initiate changes in the process parameters.